Window for infra-red detectors and method of making the same



July 29, 1969 w, HEWANN ET AL 3,458,379

WINDOW FOR INFRA-RED DETECTORS AND METHOD OF MAKING THE SAME Filed May11, 1964 Fig.1

INVE N T 095 M: Mr W m By I Nil/14" Iva/ United States Patent Int. (:1.135% 27/00 US. Cl. 156272 6 Claims ABSTRACT OF THE DISCLOSURE An openingdefined by an edge portion of a metal member is hermetically closed bysuperposing upon the edge portion a substantially plated-shaped body ofa material selected from the group consisting of potassium bromide,caesium bromide, caesium iodide and mixed crystals of thallium bromideand thallium iodide and having a melting point below the melting pointof the metal member, so as to cover the opening with the plate-shapedbody, the latter being supported by the edge portion of the metalmember, heating the edge portion of the metal member to the meltingtemperature of the body so as to melt the portion of the body contactingthe thus heated metal edge portion causing downward gravity flow ofmolten material of the body around the metal edge por- I tion so as toentrain the same and simultaneously lowering of the body into theopening, and reducing the temperature of the metal edge portion to belowthe melting temperature of the body when the body has been lowered intothe opening for a predetermined distance.

The present invention relates to windows for infra-red detectors and toa method of making the same. More particularly, the present invention isconcerned with providing an hermetic seal between the window-formingmaterial and the adjacent housing portion.

The materials which form the window panes or windows for vacuum-tight orhermetically closed infra-red detectors generally have a high thermalexpansion coefficient, such as for instance, the material known as KRSwhich is a mixed crystal of thallium-bromide and thallium iodide.Furthermore, these materials for instance, potassium bromide, caesiumbromide, caesium iodide, frequently possess only very little mechanicalstrength. In addition, these materials frequently are hydroscopic.

In view thereof, the connection between such window material and thehousing, which connection must form a hermetic seal, is frequentlyformed of organic compositions such as epoxy resins or of inorganicconnecting or binder materials such as silver chloride.

However, it is well known that organic compounds or compositions are notdesirable for forming a vacuumtight seal, because by using organiccompositions it is generally not possible to obtain absolutegas-tightness, and it is also generally not possible to obtain thedesired de gree of mechanical strength. On the other hand, usinginorganic materials as the sealing material is undesirable because thesesealing materials might react with the crystals forming the window.

It is therefore an object of the present invention to overcome the abovediscussed difliculties and disadvantages.

It is a further object of the present invention to provide a window forinfra-red detectors and a method of making the same which window can beproduced in a simple and economical manner, while simultaneouslyassuring a gas-tight seal of sufiicient mechanical strength between thewindow material and the frame or metal edge of the window opening.

Other objects and advantages of the present invention will becomeapparent on further reading of the description and of the appendedclaims.

With the above and other objects in view, the present inventioncontemplates a window portion of a housing for an infra-red detectorcomprising, in combination, a metal edge portion defining an opening inthe housing, and a substantially plate-shaped member of a materialhaving a melting point lower than the melting point of the metal edgeportion entraining the same and completely covering the opening, themember forming an hermetic seal with the entrained edge portion.

The present invention is also concerned with a method of hermeticallyclosing an opening defined by an edge portion of a metal member,comprising the steps of superposing upon the edge portion asubstantially plate-shaped body of a material having a melting pointbelow the melting point of the metal member so as to cover the openingwith the plate-shaped body, the latter being supported by the edgeportion of the metal member, heating the edge portion of the metalmember to the melting temperature of the body so as to melt the portionof the body contacting the thus heated metal edge portion causingdownward gravity flow of molten material of the body around the metaledge portion so as to entrain the same and simultaneously lowering ofthe body into the opening, and reducing the temperature of the metaledge portion to below the melting temperature of the body when the bodyhas been lowered into the opening for a predetermined distance.

Thus, according to the present invention, a suitable window panematerial is placed on top of the opening which is to be closed so thatit will rest on the thin metal edge which defines such opening. Thewindow pane material will have dimensions which are somewhat larger thanthe opening which is to be closed so that an annular edge portion of thewindow pane material will rest on the annular edge portion of the metalhousing which defines the window opening.

Suitable window pane materials includes crystals of potassium bromide,caesium bromide, caesium iodide, and mixed crystals of thallium bromideand thallium iodide known as KRSS.

The metal edge portion upon which the peripheral portion of the windowpane crystal is superposed, is then heated to at least the melting pointof the window pane material. This can be advantageously accomplished bymeans of a high frequency induction coil, in other words, by inductionheating. As soon as the metal-supported peripheral portion of the windowpane crystal starts to melt, the crystal will start by gravity to belowered into the window opening. When the crystal has penetrated thewindow opening to the desired depth, heating of the metal edge portionis interrupted, and it is thereby achieved that the temperature of themetal edge portion and of the portion of the crystal in contacttherewith, will quickly drop below the melting point of the material ofthe crystal and, upon such solidification, the crystal will notpenetrate further downwardly into the window opening.

According to the present method, the window material will melt only inthe immediate vicinity of the supporting annular metal edge portionwhich defines the opening which is to be closed. The major portion ofthe crystal, i.e., the entire crystal with the exception of theperipheral portion thereof, will remain solid and unharmed. The thusmolten peripheral portion of the crystal will, upon cooling andsolidification, form a hermetic seal with the metal edge.

Since the metal edge is formed of a very thin metal sheet, the tensionstresses which will occur upon cooling of the window pane material andthe metal, due to the different rate of reduction in the length thereofupon cooling, will be easily taken up. It is important that anyremaining tension will be directed thereby in radial symmetry towardsthe center of the window pane material. These residual tensions will beeasily taken up by the crystal forming the window pane, since theyresult only in subjecting the window pane material to a slight degree ofpressure.

It is possible, according to the present invention, to close openings infiat metal sheets, as well as the end portion of tubular members.

In order to avoid too deep a penetration of the window material into themetal or of the metal edge into the Window material, it is preferredaccording to the present invention, to place underneath the opening afiat sheet, or underneath the end portion of a tube which is to beclosed according to the present invention, a member which will supportthe window material when it has penetrated to the desired depth so thatthe window material will be prevented from further downward movementonce it has reached the supporting or stopping member.

The heating of the metal edge which defines the window which is to beclosed can be carried out in any desired manner, however, it ispreferred to carry out this localized heating by means of a highfrequency induction coil. The metal edge portion which forms or definesthe opening or window which is to be closed according to the presentinvention, should consist of one of the metals which are' suitable forforming a hermetic seal with molten material, preferably silver. Thethickness of the metal edge portion preferably will be between 0.05 and0.1 millimeter.

The crystal which is to form the window is placed on the openingpreferably so that it will extend outwardly of the window opening andwill be supported by the metal edge portion along a peripheral edge ofthe crystal having a width of between about 0.5 and 1.0 millimeter. Theoverall dimensions, i.e., the length and width, of such crystal windowpane may be of the magnitude of about 10 millimeters.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accom- I panying drawings,in which:

FIG. 1 is a cross-sectional somewhat diagramatic elevational view of anarrangement for closing an opening defined by the annular edge of atubular housing member; and

FIG. 2 is a somewhat diagramatic, elevational view in cross sectionillustrating the closing of a window opening in a flat metal sheetmember of the housing.

Referring now to the drawing, in both figures reference numeral 1denotes the crystal which serves for closing the window, while referencenumerals 2 and 22 respectively indicate the metal edge portion definingthe window opening which is to be closed. Members 3 and 23 respectivelyare arranged underneath the window opening, according to FIG. 1outwardly of the tubular member 2, and according to FIG. 2 below theflat metal sheet 22, for supporting window material 1 as soon as thesame has penetrated the window opening to the desired depth.

Heating of the edge portion 5 is carried out by means of induction coil4.

As soon as metal edge portion 5 will be thus heated to the meltingtemperature of the Window pane material 1, the window pane material willpenetrate the window edge 5, and will slowly move downwardly by gravityuntil it is supported by supporting member 3. At this moment,

i.e. as soon as contact has been established between window material '1and supporting member 3, heating is interrupted and upon cooling andsolidification of the molten peripheral portion of window material 1, anhermetically sealing bond between window material 1 and metal edgeportion 2 will be established.

In order to achieve, according to the embodiment illustrated in FIG. 2,that the molten portion of material 1 will embed the annular edge offlat sheet 22, which edge portion defines the window, it has been foundadvantageous to give to the upper face of the stopping member 23 a shapeand size somewhat larger than that of the window, and to have theperipheral edge of the upper face of holding member 23 extendingupwardly as shown in FIG. 2.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofhermetic sealing arrangements differing from the types described above.

While the invention has been illustrated and described as embodied in awindow portion of a housing for an infra-red detector, it is notintended to be limited to the details shown, since various modificationsand structural changes may be made without departing in any way from thespirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of hermetically closing an opening defined by an edgeportion of a metal member, comprising the steps of superposing upon saidedge portions a substantially plate-shaped body of a material selectedfrom the group consisting of potassium bromide, caesium bromide, caesiumiodide and mixed crystals of thallium bromide and thallium iodide andhaving a melting point below the melting point of said metal member soas to cover said opening with said plate-shaped body, the latter beingsupported by said edge portion of said metal member; heating said edgeportion of said metal member to the melting temperature of said body soas to melt the portion of said body contacting the thus heated metaledge portion causing downward gravity flow of molten material of saidbody around said metal edge portion so as to entrain the same andsimultaneously lowering of said body into said opening; and reducing thetemperature of said metal edge portion to below the melting temperatureof said body when said body has been lowered into said opening for apredetermined distance.

2. A method of hermetically closing an opening forming a window portionof a housing for an infra-red detector and being defined by an annularedge portion of a metal tube, comprising the steps of superposing uponsaid edge portion a substantially plate-shaped body of material selectedfrom the group consisting of potassium bromide, caesium bromide, caesiumiodide and mixed crystals of thallium bromide and thallium iodide andhaving a melting point below the melting point of said metal member soas to cover said opening with said plateshaped body, the latter beingsupported by said edge portion of said metal member; heating said edgeportion of said metal member to the melting temperature of said body soas to melt the portion of said body contacting the thus heated metaledge portion causing downward gravity flow of molten material of saidbody around said metal edge portion so as to entrain the same andsimultaneously lowering of said body into said opening; and reducing thetemperature of said metal edge portion to below the melting temperatureof said body when said body has been lowered into said opening for apredetermined distance.

3. A method of hermetically closing an opening form ing a window portionof a housing for an infra-red detector and being defined by an annularedge portion of a substantialy fiat sheet metal member, comprising thesteps of superposing upon said edge portion a substantially plate-shapedbody of a material selected from the group consisting of potassiumbromide, caesium bromide, caesium iodide and mixed crystals of thalliumbromide and thallium iodide and having a melting point below the meltingpoint of said metal member so as to cover said opening with saidplate-shaped body, the latter being supported by said edge portion ofsaid metal member; heating said edge portion of said metal member to themelting temperature of said body so as to melt the portion of said bodycontacting the thus heated metal edge portion causing downward gravityflow of molten material of said body around said metal edge portion soas to entrain the same and simultaneously lowering of said body intosaid opening; and reducing the temperature of said metal edge portion tobelow the melting temperature of said body when said body has beenlowered into said opening for a pre determined distance.

4. A method of hermetically closing an opening forming a window portionof a housing for an infra-red detector and being defined by an edgeportion of a metal member, comprising the steps of superposing upon saidedge portion a substantially plate-shaped body of a material selectedfrom the group consisting of potassium bromide, caesium bromide, caesiumiodide and mixed crystals of thallium bromide and thallium iodide andhaving a melting point below the melting point of said metal member soas to cover said opening with said plateshaped body, the latter beingsupported by said edge portion of said metal member; heating said edgeportion of said metal member to the melting temperature of said body soas to melt the portion of said body contacting the thus heated metaledge portion causing downward gravity flow of molten material of saidbody around said metal edge portion so as to entrain the same andsimultaneously lowering of said body into said opening; and preventingfurther downward movement of said body and reducing the temperature ofsaid metal edge portion to below the melting temperature of said bodywhen said body has been lowered into said opening for a predetermineddistance.

5. A method of hermetically closing an opening defined by an edgeportion of a metal member consisting essentially of silver, comprisingthe steps of superposing upon said edge portion a substantiallyplate-shaped crystalline body of a material being selected from thegroup consisting of potassium bromide, caesium bromide, caesium iodideand mixed crystals of thellium bromide and thallium iodide, and having amelting point below the melting point of said metal member so as tocover said opening with said plate-shaped body, the latter beingsupported by said edge portion of said metal member; heating said edgeportion of said metal member to the melting temperature of said body soas to melt the portion of said body contacting the thus heated metaledge portion causing downward gravity flow of molten material of saidbody around said metal edge portion so as to entrain the same andsimultaneously lowering of said body into said opening; and reducing thetemperature of said metal edge portion to below the melting temperatureof said body when said body has been lowered into said opening for apredetermined distance.

6. A method of hermetically closing an opening defined by an edgeportion of a metal member, comprising the steps of superposing upon saidedge portion a substantially plate-shaped body of a material selectedfrom the group consisting of potassium bromide, caesium bromide, caesiumiodide and mixed crystals of thallium bromide and thallium iodide andhaving a melting point below the melting point of said metal member soas to cover said opening with said plate-shaped body, the latter beingsupported by said edge portion of said metal member; induction heatingsaid edge portion of said metal member to at least the meltingtemperature of said body so as to melt the portion of said bodycontacting the thus heated metal edge portion causing downward gravityflow of molten material of said body around said metal edge portion soas to entrain the same and simultaneously lowering of said body intosaid opening; and reducing the temperature of said metal edge portion tobelow the melting temperature of said body when said body has beenlowered into said opening for a predetermined distance.

References Cited UNITED STATES PATENTS 4/1947 Alpert 287189.365 2/1951ProW 156273 XR 8/1953 Marshall 156-273 XR 2,744,592 5/1956 Remond287189.365 3,220,815 11/1965 McMillan et al 32 EARL M. BERGERT, PrimaryExaminer G. W. MOXON II, Assistant Examiner US. Cl. X.R.

